1. Field of the Invention
The present invention generally relates to the function of locating cellular telephones and particularly, to a method and apparatus which take advantage of the multiple channels used by a cellular telephones for this purpose.
2. Statement of the Prior Art
The proliferation of cellular telephones, or cell phones, and their technology and usage, has revealed various applications for the ability to locate cell phones. These applications include "911" calls, tourist and travel information, tracking of unauthorized cell phone usage and illegal activities and the locating of commercial and government vehicles, to name a few. The basic cell phone systems can only determine the nearest cell phone base station which locates the cell phone to within 3 to 10 miles.
A great deal of technology also exists concerning the tracking or locating of radio transmitters. A goal of any such system for use with cellular phones is the use of as much existing equipment as possible or at least compatibility with existing equipment. Examples of this are use with unmodified cellular phones and compatibility with existing cellular base stations.
One known method for locating transmitters is time difference of arrival, tdoa, which has been used for many years, at least in such applications as LORAN and GPS. The application of this process to cell phones includes measuring the time of arrival of the same signal at a multiplicity of locations and comparing the times to determine how long the signal took to reach each receiver. Because it operates on the ordinary signal transmitted, it does not require any modifications of the transmitter. One such application of tdoa technology combined with direction finding and applied to cellular phones is described in U.S. Pat. No. 5,317,323. The system described therein uses GPS timing signals at the receivers to determine the time of signal reception and generally concerns the use of direction finding to help eliminate multipath and co-channel interference. Unfortunately, the use of direction finding requires the use of a steerable antenna array and is thus not compatible with existing omnidirectional antenna structures. The patent does show the importance of removing multipath interference. For this reason, it is a goal of any such locating system for cellular telephones to distinguish between a direct signal from a cell phone and multipath reflections of the same signal from buildings and other reflectors. Reflected signals, which take a longer and unknown path to the receiver, provide less accurate location information.